226 Basic principles for molding land mosaics
vegetational density and species richness, affect adjacent habitats by
functioning as a source of effects and as a filter of movements between
thehabitats.
(H) Degradation of a natural community or ecosystem. Degradation by human
activity reduces vertical and horizontal structure, such as foliage layers,
tree holes, vegetation gradients, and soil horizons, and reduces func-
tional interactions and flows, including food webs, water flows, and
mineral nutrient cycles.Natural habitats for conservation
(A) Number oflarge patches.Consistent with risk-spreading theory, if each
large patch of a particular habitat type contains almost all of its char-
acteristic species in a landscape, then two or three large patches are
probably sufficient to sustain almost all the species, but if each patch
has a limited portion of the characteristic species present, four or five
large patches are probably required.
(B) Especially valuable patches.Natural vegetation patches that play a particu-
larly important role in the overallsystem (such as a key link in the land-
scape pattern), or contain unusual or distinctive characteristics (such as
an important aquifer or rare habitat), are especially valuable for mini-
mizing degradation.
(C) Economically productive areas.Remnant natural habitats in particularly
productive areas especially merit habitat expansion, because they tend
to be rare and tocontain many rare species that thrive on the rich
environmental conditions.
(D) Habitat diversity.Increasing the number of habitat types, primarily by
including more substrate and microclimatic conditions or secondar-
ily by maintaining more successional stages (e.g., fallow fields, shrubby
areas), increases the number of native species present.
(E) Tree holes and dead wood.Dead wood, both standing and fallen, and cavi-
ties in tree trunks tend to be scarce in built areas, yet are especially
important for biodiversity benefits.
(F) Rare and representative habitats.Byprotecting reasonable numbers and
sizes of rare and representative habitats, nature (including the bulk of
thenative species present) should persist long term.
(G) Asmall isolated habitat.Toprotect a small isolated habitat long term
typically requires the presence of an important role played by the habitat
within a larger landscape pattern, and may also require widespread
public recognition.
(H) Ecology, cost, and threat.Successful long-term land protection particu-
larly focuses on location of the land relative to other protected lands,